For a new set of communication skills that can really make a difference, you might want to google Non-Violent Communication. I've been applying a modified version of this with children that I've worked with over recent years, including a 4yo grandson, and it is quite effective. It teaches you to think in terms of feelings and needs, rather than concepts and issues. It is extremely respectful for all parties in the process of communication, and can help ease us out of situations that seem hopelessly stuck.
You don't say, but I'm guessing there is a particular constellation of issues that you and your children differ over, and that when you act as though they should listen to/obey your wishes, the fur starts flying. (If this isn't the case, perhaps you can provide more information for the rest of us to respond to.)
It's a hard thing to learn, if we haven't made a habit of this since their infancy, but for me it seems it must start with you respecting them, and yourself. At some point it will serve you all if you can see them as adults, and treat them as respectfully as you would any other adult. Amazingly, once they believe you're sincere, they are likely to start treating you with more respect. Old habits die hard, though. It could take continued effort on your part, and hits and misses on their part.
Of course, there are plenty of dynamics that could complicate things. If you are truly worried about some of the choices your children are making, for example, then of course you'll feel upset and angry when they defy your wishes. But I suspect that the core issue is still whether you're willing to see them as adults now. You've had 18 or more years to guide your children into responsible, caring adulthood. It might be time to let loose.
And not infrequently, once parents lay down old power struggles, the kids are free to find their own answers. And they often do. I've seen it happen a lot in my 60-some years, and have experienced that myself when I finally got out from under my own mother's control.
My best to you, D.. This is not an easy issue. But it's a great opportunity for all of you to grow in maturity.